In the field of pressure sensitive adhesive (PSA) compositions, ongoing technological developments in the coating technique mean that there is a progressive need for new developments. Within the industry, hotmelt processes with solventless coating technology are of increasing importance in the preparation of PSA compositions, since the environmental regulations are becoming ever greater and the prices of solvents continue to rise. The consequence of this is that solvents are to be eliminated as far as possible from the manufacturing process for PSA tapes. The associated introduction of the hotmelt technology is imposing ever greater requirements on the adhesive compositions. Acrylic PSA compositions in particular are the subject of very intensive investigations aimed at improvements. For high-level industrial applications, polyacrylates are preferred on account of their transparency and weathering stability. In addition to these advantages, however, these acrylic PSA compositions may also meet stringent requirements in respect of shear strength and bond strength. This profile of requirements is met by polyacrylates of high molecular weight and high polarity with subsequent efficient crosslinking. For the crosslinking there are in principle two methods available, since the thermal crosslinking of acrylic hotmelt PSAs can be realized only by circuitous routes. Electron beam crosslinking offers the advantage that crosslinking can still be carried out at relatively high adhesive application rates. The disadvantage is the inefficient crosslinking, since there are no reactive groups, such as double bonds, for example. Consequently, the quality of electron beam crosslinking of polyacrylates is always relatively poor.
UV crosslinking requires less elaborate apparatus and is therefore of advantage for industrial application. In this case, free-radical intermediates are formed which react with one another and lead to crosslinking of the PSAs.
U.S. Pat. No. 4,717,605 describes a method of adhesively bonding optical glass components. It does so using radiation-curable adhesives based on ionically polymerizable epoxy systems and ionic photoinitiators based on triarylsulfonium complex salts. These adhesives still contain at least one ethylenically unsaturated compound, which can be polymerized by free radicals in the presence of a free-radical photoinitiator.
WO 88/02879 uses a photoinitiator and an iron salt for cationic photopolymerization. That document discloses a polymerizable composition composed of a free-radically polymerizable material and a photocatalyst. The photoinitiator system is composed of π-aromatic-metal complexes of the form(R6—Fe+—R7)LZk−,where R6 is an η6-aromatic, R is the anion of a cyclopentadienyl compound, L is a di- to heptavalent metal or metalloid, Z is a halogen, and k is the valence of L increased by 1. The photocatalyst system further comprises a peroxidic or hydroperoxidic compound and, optionally, a metallocene.
U.S. Pat. No. 5,776,290 claims a method of preparing a coated abrasive article, where a first binder is present on a backing and a multiplicity of abrasive particles are present in this binder. The binder is composed of a pressure sensitive hotmelt adhesive film. This hotmelt adhesive is cured by means of an energy source so that the abrasive particles are covered by a crosslinked coat of the adhesive. The hotmelt adhesive crosslinked with an epoxide in this document is based on pressure sensitively adhesive polyesters.
The UV-initiated epoxide crosslinking reaction, as a highly efficient crosslinking reaction, has not been successfully transferred to date to polyacrylate PSAs.
It is an object of the invention to provide a process for crosslinking polyacrylates, especially polyacrylate-based hotmelt adhesives, which does not have the disadvantages of the prior art and which advantageously extends the state of the art.